We introduce a generic type of observational signals originated from the primordial universe that can be used to model-independently distinguish the inflation scenario from alternatives. We use oscillating massive fields as the primordial standard clocks. These fields exist in any models and their oscillations are excited either classically or quantum mechanically in the primordial universe. These oscillations induce features in the primordial density perturbations that directly record the scale factor of the primordial universe as a function of time. If detected, such signals would provide a direct evidence for the inflation or an alternative scenario. The amplitudes of such signals, however, are very model-dependent.